In a 3rd Generation Partnership Project (3rd Generation Partnership Project, 3GPP) Long Term Evolution (Long Term Evolution, LTE) or LTE-advanced (LTE-advanced, LTE-A) system, a downlink multiple access manner generally adopts an orthogonal frequency division multiplexing multiple access (Orthogonal Frequency Division Multiple Access, OFDMA) manner. A downlink resource of the system is divided into an orthogonal frequency division multiplexing multiple access (Orthogonal Frequency Division Multiple, OFDM) symbol in terms of time, and is divided into a sub-carrier in terms of frequency.
According to an LTE Release 8/9/10 (LTE Release 8/9/10) standard, a normal downlink subframe includes two time slots (slot), each time slot has 7 OFDM symbols, and a normal downlink subframe has a total of 14 or 12 OFDM symbols. The LTE Release 8/9/10 standard also defines a size of a resource block (Resource Block, RB), where an RB includes 12 sub-carriers on a frequency domain, and is of a duration of a half subframe (a time slot) on a time domain, that is, 7 or 6 OFDM symbols are included. On a subframe, a pair of RBs of two time slots is called a resource block pair (RB pair, RB pair). In actual transmission, a resource block pair used on a physical resource (a physical RB pair) is also called a physical resource block pair (Physical RB pair, PRB pair). A PRB pair is generally called a PRB for short, so that in the following description, a PRB, a PRB pair, a physical resource block, and a physical resource block pair all refer to a PRB pair.
Various data borne on a subframe is mapped by dividing a physical time-frequency resource of the subframe into various physical channels. The various physical channels are mainly classified into two types: control channels and service channels. Correspondingly, data borne by a control channel may be called control data (or control information), and data borne by a service channel may be called service data. A fundamental purpose of sending a subframe is to transmit the service data, and a function of the control channel is to help transmission of the service data.
A complete physical downlink control channel (Physical Downlink Control Channel, PDCCH) is formed by one or several control channel elements (Control Channel Element, CCE). According to the LTE Release 8/9/10, a PDCCH may be formed by 1, 2, 4, or 8 CCEs, which correspond to aggregation levels 1, 2, 4, and 8 respectively.
In an LIE system, because of introduction of technologies such as multiple user multiple input multiple output (Multiple Input Multiple Output, MIMO) and coordinated multiple points (Coordinated Multiple Points, CoMP), control channel capacity is limited, so that a PDCCH transmitted based on an MIMO precoding manner (called an ePDCCH below) is introduced. The ePDCCH may be demodulated based on a UE-specific reference signal-demodulation reference signal (Demodulation Reference Signal, DMRS).
For the ePDCCH, each ePDCCH is still formed by k logical elements (defined as an eCCE herein) similar to the CCE, and a user equipment needs to perform blind detection on a terminal side. Following a definition of the aggregation level in the PDCCH, an ePDCCH with the aggregation level being L (L=1, 2, 4, 8) is formed by L eCCEs.
Manners in which a base station transmits the ePDCCH include two transmission manners, namely, centralized or localized (localized) and distributed (distributed) transmission manners. In the centralized or localized transmission manner, control channels of the ePDCCH may be allocated to consecutive time-frequency resources, and meanwhile the base station sends the ePDCCH to a UE by using beamforming or precoding, so as to obtain a beamforming/precoding gain. In the distributed transmission manner, control channels of the ePDCCH may be allocated to discrete time-frequency resources, so as to obtain a (frequency) diversity gain.
For the centralized or localized transmission manner, before sending the ePDCCH to the user equipment, the base station first configures a search space, that is, a PRB pair, for the to-be-transmitted ePDCCH, and 4 eCCEs may be placed in each PRB pair. Then, for the to-be-transmitted ePDCCH, one of candidate positions is selected from front to back according to an arrangement order of the PRB pair to place the ePDCCH to perform transmission, which is not conducive to better sending of control information.
For example, for an ePDCCH that bears control information and has an aggregation level of 1, the base station configures a total of four PRB pairs including a PRB pair 3, a PRB pair 4, a PRB pair 8, and a PRB pair 9. According to the foregoing method, the base station transmits the ePDCCH only on the PRB pair 3 and the PRB pair 4. Even if the base station learns, through feedback of the UE, that channels of the PRB pair 8 and the PRB pair 9 are better, the ePDCCH with the aggregation level being 1 cannot be sent on the PRB pair 8 or the PRB pair 9.